This invention relates, in general, to a method for refining molten metals or alloys. Specifically, the invention relates to the particular step of decarburizing metals or alloys, especially stainless steel, carbon steel, low carbon steel, iron, nickel and cobalt based alloys.
It is known from the work of Savard et al., U.S. Pat. No. 2,855,298, that injection of gases, through a tuyere, below the surface of a molten metal in a containing vessel is one method for refining the molten metal. In particular this method is used for refining iron, steel, stainless steel and zinc. The method uses high pressure oxygen, which has a localized cooling effect on the submerged tuyere, to penetrate the bath and affect decarburization.
Nelson et al., U.S. Pat. No. 3,046,107, and later, Krivsky, U.S. Pat. No. 3,252,790, introduced methods for decarburizing metal baths, without substantial loss of chromium. These methods are known as the argon-oxygen decarburization "AOD" process. The "AOD" process was developed because molten stainless steels containing desirable amounts of chromium could not be decarburized without severe oxidation of the chromium. In the "AOD" process, a molten metal is decarburized by subsurface blowing with an inert gas-oxygen mixture. The presence of the inert gas, usually argon, reduces the partial pressure of carbon monoxide formation in the ga in contact with the metal. This operation results in the oxidation, and thus removal, of carbon preferentially to the oxidation of chrome.
Later, Heise et al., U.S. Pat. No. 3,861,888, disclosed a method which adds CO.sub.2 to an argon-oxygen mixture to form a three-gas component mixture for decarburizing metals.
It now has been found, in accordance with the invention, that argon can be completely replaced by carbon dioxide and a two-gas component mixture used to effect decarburization. Additionally, it has been found that the varying stages of decarburization cannot properly be treated equally as one single process. Each stage of decarburization is differently affected by many variables including the original carbon content of the molten metal, oxygen flow rate, carbon dioxide flow rate, furnace condition, temperature of the injection gases, bath temperature and aim temperature of the melt. By understanding the effect of the many variables on the different stages of decarburization, it is possible to improve the carbon removal efficiency in decarburization of molten metals and alloys.